A gasket having a cross sectional shape longer than it is wide and is prevented from falling over and buckling. The gasket is installed in an installation groove formed in either of two opposite surfaces and having a cross sectional shape that is longer in the depth direction of the groove than it is wide. The gasket has a meandering section meandering in the width direction of the installation groove and also has a rib provided projecting from the inner periphery and/or the outer periphery of the meandering section.